This invention relates generally to apparatus and methods for determination of pipe length. Specifically, the invention relates to an apparatus and method for determining the length of pieces of pipe by employing sonic waves.
Accurately determining the length of pieces of pipe is very important in many industries. In the oil industry, for example, standard thirty-foot lengths of pipe are threaded and used in the drilling of oil wells. Upon reuse the worn threads or couplings may be cut off, thereby shortening the length of pipe. The price of pipe and installation thereof is based on a dollar amount per linear foot. Therefore, an accurate determination of the length of pipe is needed for payment to the oil drilling operator. In many other industries such as irrigation, well drilling, and steam fitting the length of pipe used is also very important.
In the oil industry, it is common practice to remove from the ground and store on end approximately 90 foot sections of pipe on an oil drilling platform A 90 foot section is composed of three 30 foot sections coupled together. What is needed is a method to determine exactly the length of removed pipe. One method that has been tried is the use of a laser. This necessitates the placement of a mirror at one end of a pipe and a apparatus containing a laser at the other end and a means for determining the time of flight for light to pass from one end of the pipe to the receiving unit at the same end of the pipe as the laser unit These units have not been successful because of their difficult use and high cost. It requires a placement of a mirror at one end of the pipe which is not possible with pipe standing vertically, and even when sections are laid horizontally, it does not permit ease of operation by one person.
Various schemes have been tried to sonically measure the length of pipe such as the method disclosed in U.S. Pat. No 4,241,430 to Kayem et al. The Kayem et al Patent discloses the use of a source/detector which mechanically couples to the pipe to be measured and sends out an acoustic signal. The teachings of Kayem et al are herein incorporated by reference. The sound echo returning from the far end of the pipe is sensed and based on its time of travel gives a measurement of the pipe length. Compensation for the speed of sound is made by measuring the air temperature. This causes large inaccuracies due to the difference between measured temperature of the air versus the actual temperature of the air in the pipe. In addition, relative humidity and barometric pressure factors are not accounted for in the Kayem et al invention. The apparatus as disclosed does not take into account any difference in the pipe being open-ended or closed-ended. As mentioned earlier, in an oil drilling operation, oil pipe is often stored on a drilling platform having a wooden or metal floor The pipe is stored vertically on end on the platform and this effectively makes for a closed end condition.
U.S. Pat. No. 4,584,676 to Newman is directed to a method and apparatus for pipelength measurement which tries to correct for some of the inaccuracies which were not addressed in the Kayem et al Patent. Newman recognizes the use of mechanical compensation for open-ended pipe measurement. The teachings of Newman are herein incorporated by reference. The open air coupling method he uses is prone to receive ambient noise such as passing trucks, drilling rig operation and other sound waves which may give rise to false readings. No accurate method of determining the speed of sound is made other than temperature measurements. The amplification and detection system is not able to handle problems of pipe couplings near the source or connectors within a string of pipe, all of which may give rise to false echoes.
What is needed for practical application is a pipelength detection system capable of measuring pipelength length to the nearest 1/4 inch in 90 feet under dirty conditions with multiple pipe sections, including couplings at the source end of the pipe. The system must be able to measure both open-ended and closed-ended pipes and be able to do so under real world operating conditions.